It has to be constantly repeated that the existence of a particular feature or function in a modern-day animal proves little about the anatomy of that animal 600 million years ago, which is more or less when the brain first began to evolve. The direct knowledge that we have about the brain anatomy of ancient animals (our ancestors) is based partly on fossils, which consist mostly of hard tissue – the soft brain tissue has gone – but increasingly on DNA and mitochondrial analysis. While the succession of types of animal in our ancestral tree is reasonably secure, the assertions in the following sections about differentiation of the brain in early animals, even though based on generally accepted current levels of knowledge, are hardly better than informed guesswork, particularly as regards the functions of early divisions of the brain.
Pretty much everyone knows that the brain stem connects the spinal cord to the brain proper. It deals with highly instinctive survival functions including breathing, digestion, heart rate, and blood pressure; and controls many reflex motor responses. The brain stem includes the reticular formation, which is essential to consciousness and plays a major role in arousal (being awake and alert). The brain stem receives many types of sensory input and 'pre-processes' it before sending it on to higher parts of the human brain. The top section of the brain-stem is called the pons (bridge).
A primitive brain stem, termed a notochord, had already evolved about 600 million years ago (MYA) in our ancestors the Protostomes (modern-day example, the ragworm, see Chapter One), while longtitudinal nerve-cords existed in their own probable ancestor the Acoeli (flatworms), which are assumed to have had a fairly undifferentiated ganglion (collection of neural cells) towards the front part of the animal, and are the first known bilaterally symmetrical animals, although still invertebrate.
At a considerably later stage of brain evolution, the cerebellum (see below) developed alongside the pons. The brain-stem and the cerebellum are known together as the hind-brain.
The Three-Part Brain
The three-part brain consists of the fore-brain (prosencephalon), the mid-brain (mesencephalon) and the hind-brain (rhombencephalon). In early types of animal such as the ragworm, the fore-brain is chiefly concerned with the acquisition of food, using an olfactory sense, while the mid-brain processes optical input, perhaps mostly for reproductive purposes consequent upon particular external conditions of luminosity, and the hind-brain deals with control of motor activity.
The ragworm already displays the three part arrangement of early animal brains, which was conserved to a considerable extent in subsequent brain evolution, and this arrangement is very clearly present in the hagfish, similar to our probable last pre-vertebrate ancestor (evolved c. 530 MYA, see Chapter One), and the lamprey (evolved c. 500 MYA), similar to probably our first vertebrate ancestor.
During embryonic development, in more advanced animals, the fore-brain divides into two sections, the cortex (telencephalon), destined to be the seat of intelligence and consciousness, and the set of regions dealing with emotion and other bodily drives (known as the diencephalon) as well as communication between the various brain regions.
The evolutionary origins of the mid-brain, which sits on top of the pons at the head of the brain-stem and is known as the tectum (roof), are murky. The tectum consists of the superior colliculus and the inferior colliculus. It receives visual and auditory sensory input; in humans it is involved in only preliminary sensory processing, but in non-mammalian vertebrates it serves as the main visual area of the brain, functionally analogous to the visual areas of the cerebral cortex in mammals. It seems possible that in early animals, the eyes had a mostly reproductive function, judging the best moment for release of gametes based on prevailing light conditions (always in the sea, at that time, of course). It is thought that the visual sense thus evolved on top of the animal (dorsally) in distinction to the brain stem and spinal cord, which were originally underneath (ventral). Many writers suppose that during the period when Protostomes were evolving, the animal 'flipped over' (eg Telford, 2007) so that the nerve-cord moved to the top, and that may have been the moment at which the visually-oriented structures of the mid-brain joined up with the brain-stem (hind-brain).
In modern animals with a cortex, the thalamus, which has a bi-lobed structure – the precursor of the two hemispheres of the developed mammal brain – is the gateway for all incoming sensory information on its way to the cortex. In early animals, such as the hagfish, which did not yet have a clearly defined cortex, such sensory integration as took place happened in or close to the thalamus, and it must also be the place where motor commands originated as a result. The hypothalamus, which is strongly connected to the thalamus and sits just underneath it (behind it, in the case of the eel-like hagfish) controls the endocrine system, the chemical messenger system of the body, which operates in conjunction with the electrical, neural messenger system. Chemical messengers are of course produced by glands, of which the pituitary gland, physically almost a part of the brain, was one of the earliest. The hagfish has (and is presumed to have had) a thalamus, a hypothalamus and a pituitary gland.
diagram of the human brain showing how the thalamus sits on top of the
Copyright Eileen Nicole Simon; http://www.conradsimon.org
Chemical messengers were and are a means of communicating affective (sometimes called hedonic) states to the body at large. In higher animals, the housing and implementation of affective states becomes a function of parts of the cortex (see below) and employs neural mechanisms as much as or more than chemical messengers; but in less developed animals affective agendas are a matter for the thalamus to deal with and are implemented mostly in chemical terms.
Even in modern versions of the hagfish and lamprey, there is relatively little communication between the several parts of the brain, although in both species there is a well-develope thalamus, and the fore-brain has acquired a new Extra Pyramidal System (EPS) that deals with involuntary motor commands, linked to an early version of the amygdala, permitting the expression of positive or negative haptic states (reward and avoidance) through motor activity. The EPS is so called because it came into existence before appearance of the system based on pryamidal neurons that underpins voluntary motor actions under the control of the cortex in more modern animals.
The cerebellum, which sits below the mid-brain at the top of the brain-stem, is mostly concerned with the fine control of movement, although it has been accredited with a number of other functions in higher mammals. While some cerebellar-like structures have been described in hagfish and lampreys, their role is disputed, and the consensus is that the cerebellum evolved first in post-lamprey vertebrates, after which it became universal in vertebrates, and has tended to become a proportionately larger part of the brain with advancing cortical sophistication. It receives input primarily from the brain-stem, and outputs mostly through the thalamus to the cortex.
The ability of the hagfish to tie itself into a knot and ease the knot up or down its length, assisted by its slimy surface, is the sort of wave-like succession of motor actions which might be thought to require a coordinating ability from the brain – just what the cerebellum evolved to provide; but if there is a cerebellum-like function in the hagfish brain, it is very undeveloped (Kusunoki et al, 1982).
This word is used to describe the 'fore-brain', whose main components are the cortex (cortical hemispheres), the amygdala, the hippocampus and the basal ganglia (whose most prominent component is the striatum). Very approximately, the functions of these components of the modern mammalian brain are as follows:
The hypothalamus, amygdala, and hippocampus are together described as the limbic system, being associated with emotions such as fear and pleasure, memory, motivation, and various autonomic functions. The idea of a limbic 'system' as such has somewhat gone out of fashion, as it becomes apparent that the interconnections of the mammalian brain are too complex to permit of a separate system as such for affective or hedonic purposes.
Just as the mid-brain, with its visual function, probably evolved separately from the brain-stem (with its motor function) and joined up with it 550 million years ago, so it is supposed that the fore-brain, with its olfactory function, evolved separately and joined up with the mid-brain and hind-brain round about 500 million years ago. The olfactory sense probably evolved in connection with the detection of food, and some writers link the development of memory with the need for a data-base of information about the characteristics of food sources. Perhaps this idea gels with the power of smells to arouse very deep-seated and long-ago emotions.
The hagfish (from c. 530 MYA) is both invertebrate and jawless, and was succeeded by the lamprey (from c. 500 MYA), vertebrate but still jawless. These animals display increasing levels of connectivity between the various sections of the brain, and both have a primitive amygdala-striatum-hippocampus (diencephalon) linked to their olfactory sense. However it is then a big step forward to the next group of animals which can securely be tied back to our evolutionary tree, the Chondrichthians (eg the shark), which are jawed vertebrates and evolved about 460 MYA.
The emergence of jawed animals, probably between 480 and 500 million years ago, can perhaps be associated with greater differentiation of the parts of the fore-brain. By 450 million years ago, two types of jawed fish had emerged: plate-skinned archaic jawed fishes (cartilaginous fishes including sharks and rays) and bony jawed fishes (known as Placoderms). Although it's the latter that are in the direct line of our ancestry, the likelihood is that the two types had a common ancestor, and among existing fish it is probably the sharks that most closely rememble such an ancestor.
Although the mammalian brain is folded over on itself, making a ball, the sorts of long, marine animal which evolved during the period from 450-500 million years ago display the spatial succession of fore-brain/mid-brain/hind-brain very clearly. Fore-brain (to do with finding food and eating it) at the front, mid-brain (to do with eyes on the top of the head, initially more for reproductive purposes than for seeing prey) in the middle, and hind-brain (controlling the body via the spinal cord) at the back.
By the time that jawed fish had emerged, approximately 460 million years ago, this arrangement had become standard, and is well exemplified in the brain of the dogfish shark, shown below.
About 430 million years ago, the lobe-finned fish (Sarcopterygians) evolved; these bony fish, with lobed, paired fins, are just about universally accepted as the ancestors of all tetrapods, i.e. four-limbed animals, which of course includes us humans, and their modern representatives, including the lung-fish and the famous coelacanth, which are thought to have conserved their original features to an unusual extent. The Sarcopterygians have rather small brains, but they exhibit all the features described above for the shark, and there is (disputed) evidence that the hippocampus has become differentiated.
In Amphibians, however, which evolved about 380 million years ago, there is no doubt that the hippocampus exists, although it is sometimes called the medial pallium (Gonzalez, 2002). As in so many other cases, the actual amphibian ancestor species has disappeared except in fossil form, but it is thought that its general plan has largely been conserved in modern amphibians, so that there is a fair degree of consensus on how the brain of the ancestral amphibian would have appeared.
The major divisions of the amphibian brain are essentially the same as in reptiles and mammals, although mammalian complexity is much greater, especially of course in the cortex (termed the dorsal pallium in amphibians and reptiles), which has a 6-layer construction in mammals rather than the 3-layer structure of the amphibian brain. There are however already a growing number of the re-entrant connections (feedback loops) which are such a marked feature of the mammalian brain. Herrick (The Brain of the Tiger Salamander) argues that these circuits serve to link the animal's internal motivations with its sensory and motor apparatus. One of the roles of these early additions to the simpler brains of the amphibians which preceded the Sauropsid ancestor is to mediate the interplay between emotions (already in existence through the amygdala) and what one can only call social behaviours, including cooperation, aggression, mating, rearing and teaching of young.
As compared with fish brains, the amphibian cortical hemispheres (dorsal pallium) have grown in importance and there is much more connectivity with the now very pronounced optical lobes of the mid-brain and directly with the brain-stem. On visual inspection, there appears to be far more integration between the olfactory bulbs of the fore-brain and the mid-brain; for the first time, the brain looks like one integrated body rather than separate bodies which have become connected.
A clearly differentiated cortex (still known as the pallium) makes its first appearance in the Sauropsids, a phylum which includes turtles, reptiles, lizards and birds. They are of course verterbrates (jellyfish and worms are invertebrates). It is not too clear what our original Sauropsid ancestor may have looked like, but it is a good guess that it appeared about 300 million years ago. Among extant Sauropsids, turtles are often said to be closest to the original ancestor, and snakes are quite remote.
With the evolution of reptiles the dorsal pallium grew in importance and size, reflecting the increased complexity of motor behaviour on dry land and the wealth of sensory information being delivered by expanded olfactory lobes. The part of the pallium known as the basal ganglia, and particularly the region called the striatum, developed a specialized role in the control of motor behaviour. The amygdala, hippocampus and basal ganglia, which had been linked in earlier animals, became more clearly differentiated, indeed separated by the growing pallium (Herrick, ibid; Carey, 1982).
It used to be thought that there were major structural differences between the mammalian and Sauropsid brains in terms of the cortex. It is fairly well accepted by now, however, that the mammalian neocortex and the equivalent Sauropsid (reptile) dorsal cortex and dorsal ventricular ridge (DVR) developed from a common antecedent, being the pallium, in the ancestor reptile that evolved from amphibians (eg Reiner, 2000; Husband and Shimizu, 2001). Recent research has show that functionally and biochemically there is a lot of similarity between mammalian and Sauropsid cortices. See Sanides (1969) and Deacon (1990).
Kusunoki, T, Kadota, T and Kishida, R (1982) Chemoarchitectonics of the Brain Stem of the Hagfish, Eptatretus Burgeri, with Special Reference to the Primordial Cerebellum, Hirnforsch., 23(1), pp 109-19
Loonen, A J M and Ivanova, S A (2015) Circuits Regulating Pleasure and Happiness: the Evolution of Reward-Seeking and Misery-Fleeing Behavioral Mechanisms in Vertebrates, Front Neurosci. 2015; 9: 394. Published online 2015 Oct 23. doi: 10.3389/fnins.2015.00394 PMCID: PMC4615821
Reiner, A J (2000) A Hypothesis as to the Organization of Cerebral Cortex in the Common Amniote Ancestor of Modern Reptiles and Mammals, Evolutionary Developmental Biology of the Cerebral Cortex, No. 228, Novartis Foundation
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